In the manufacture of devices useful in microelectronics, there sometimes arises a need for patterning a film of a material that is difficult to etch in a fine pattern, such as one involving line widths narrower than 2 microns. In such instances it is useful to employ a lift off process for patterning the film.
The basic principles of the lift off process for patterning thin films are set forth in a paper in Semiconductor International by J. M. Frary and P. Reise and entitled "Lift off Techniques For Fine Line Metal Patterning," pages 72 88, December, 1981.
In a lift off process, a continuous masking layer is first deposited on a desired substrate and then patterned, typically photolithographically, to form openings that expose the substrate essentially in the pattern desired for the thin film. The thin film is then deposited over the resulting mask and the exposed portions of the substrate. Finally, the mask is removed, lifting off the thin film where it has deposited on the mask, but leaving it in place where it has deposited on the exposed substrate.
A basic requirement of the lift-off process is that the deposited film not achieve step coverage and not be continuous across the openings in the mask. The discontinuity in the thin film will distinguish the areas of the film that are removed from those left behind to form the desired pattern. Additionally, the discontinuity is important to facilitate entry of the etchant or solvent used in the removal selectively of the mask.
In a practical process the lift off mask needs to have certain characteristics. It generally needs to be thicker than the thin film to be patterned and should be readily patterned to the resolution desired for this pattern. Additionally, it should be easily removed selectively, without affecting the thin film areas left behind on the substrate.
Additionally, for best results, the film should be deposited in a manner that little coats the sidewalls of the openings in the mask, since coatings on the sidewalls may provide unwanted topography that complicates further processing. Usually this is achieved by depositing the film in a directional manner. However, in certain instances the film to be deposited is of a material that is not readily amenable to directional deposition, as is the case with indium tin oxide (ITO) that needs to be sputtered on to insure the proper stoichiometry needed for proper electrical conductivity and high light transmissivity for its usual role as a light transmissive electrode in a photosensitive device.
In such instances where non-directional deposition is used, it is especially important to utilize a suitable profile in the openings in the mask to avoid sidewall deposits and to insure discontinuity of the film. As used herein, "non-directional" describes a deposition process in which significant numbers of the particles being deposited are directed at a desired target at angles u greater than about 15 degrees from the average depositing direction which usually is normal. Typical lift off profiles are discussed in the aforementioned paper and typically involve either a capped or reverse slope profile. However, achieving a suitable profile can be difficult and the present invention involves a technique for achieving a particular lift off profile essentially favorable for non-directional deposition.
As is discussed in the aforementioned paper, one technique for achieving a suitable lift off profile for the mask is to use a mask that comprises two layers, a bottom assisting layer proximate the substrate and a top photoresist layer over the assisting layer. The top photoresist layer is adapted for high resolution patterning while the bottom layer is not. In particular, the patterning of the mask is done in a manner to cause the bottom layer to be undercut sufficiently that the sidewalls of the openings formed in the bottom layer are displaced significantly from the openings formed in the top layer. As a result sidewall coating of the bottom layer is readily avoided even when the film is deposited in a non-directional fashion, as by sputtering.
In the above described paper, among the lift off processes discussed, are some that employ polyimides as the assisting layer in which undercutting is achieved during the photoresist development, since these materials advantageously have an etch capability that is compatible with photoresist processing. However, achieving the desired degree of undercutting appears to have been difficult and to require relatively complex processing.
In particular, no simple lift-off process appears to have been available to achieve films of high resolution, e.g. line widths narrower than 2 microns, of shallow thickness, e.g. films no thicker than 2000 Angstroms, and of a material that needs to be sputtered for deposition, e.g. indium tin oxide. For forming such films by a lift off process, it is important to provide in the openings of the mask an aspect ratio of at least one, where such ratio is the length of undercut to the thickness of the assisting layer being undercut. However, to achieve such an aspect ratio with thick assisting layers requires a large amount of undercutting that militates against achieving fine patterns.